The present invention relates generally to a method for exposing a tunnel barrier layer of a tunnel junction device to ultra-violet light through an overlayer that covers the tunnel barrier layer. More specifically, the present invention relates to a method for exposing an electrically non-conductive tunnel barrier layer of a tunnel junction device to ultra-violet light through an overlayer that covers the tunnel barrier layer in order to heal out defects in the tunnel barrier layer.
Magnetic tunnel junctions (MJT) are devices that include two ferromagnetic (FM) layers of material that are separated by a thin dielectric layer (i.e. an insulating layer) which acts as a tunnel barrier. Uses for tunnel junction devices include magnetic field sensors and thin film high density read and write heads for hard disk drives. Magnetic Random Access Memory (MRAM) is an emerging technology that also incorporates a tunnel junction and can provide an alternative to traditional data storage technologies. MRAM has desirable properties such as fast access times like DRAM and non-volatile data retention like hard disk drives. MRAM stores a bit of data (i.e. information) as an alterable orientation of magnetization in a patterned thin film magnetic element that is referred to as a data layer, a storage layer, a free layer, or a data film. The data layer is designed so that it has two stable and distinct magnetic states that define a binary one (xe2x80x9c1xe2x80x9d) and a binary zero (xe2x80x9c0xe2x80x9d). Although the bit of data is stored in the data layer, many layers of carefully controlled magnetic and dielectric thin film materials are required to form a complete magnetic memory element. One prominent form of magnetic memory element is a spin tunneling device. The physics of spin tunneling is complex and good literature exists on this subject.
In FIG. 1a, a prior MRAM memory element 101 includes a data layer 102 and a reference layer 104 that are separated by a thin tunnel barrier layer 106. Typically the tunnel barrier layer 106 is a thin film with a thickness that is less than about 2.0 nm. In a tunnel junction device such as a tunneling magnetoresistance memory (TMR) the barrier layer 106 is an electrically non-conductive dielectric material such as aluminum oxide (Al2O3), for example. The tunnel barrier layer 106 is an insulator through which a tunneling current passes. The magnitude of the tunneling current and the quality of the tunneling current are greatly influenced by the quality of the insulator used for the tunnel barrier layer 106. As a voltage that is applied across the tunnel barrier layer 106 is increased, the tunneling current increases in a nonlinear fashion.
The reference layer 104 has a pinned orientation of magnetization 108, that is, the pinned orientation of magnetization 108 is fixed in a predetermined direction and does not rotate in response to an external magnetic field. In contrast the data layer 102 has an alterable orientation of magnetization 103 that can rotate between two orientations in response to an external magnetic field. The alterable orientation of magnetization 103 is typically aligned with an easy axis E of the data layer 102.
In FIG. 1b, when the pinned orientation of magnetization 108 and the alterable orientation of magnetization 103 point in the same direction (i.e. they are parallel to each other) the data layer 102 stores a binary one (xe2x80x9c1xe2x80x9d). On the other hand, when the pinned orientation of magnetization 108 and the alterable orientation of magnetization 103 point in opposite directions (i.e. they are anti-parallel to each other) the data layer 102 stores a binary zero (xe2x80x9c0xe2x80x9d).
The data layer 102 and the reference layer 104 serve as electrodes of a tunnel junction device which allow the state of the bit stored in the data layer 102 to be sensed by measuring a resistance across the data layer 102 and the reference layer 104 or a by measuring a magnitude of the aforementioned tunneling current. Although the reference layer 104 is shown positioned below the tunnel barrier layer 106, the actual position of the data layer 102 and the reference layer 104 will depend on the order in which they are formed in a process for fabricating the magnetic memory cell 101. Accordingly, the data layer 102 can be formed first and the tunnel barrier layer 106 formed on top of the data layer 102.
Ideally, the tunnel barrier layer 106 of a tunnel junction device is flat and has a uniform thickness T throughout its cross-sectional area. Moreover, an ideal tunnel barrier layer 106 would be made from a dielectric material that is homogenous. One of the criteria for an ideal tunnel barrier layer 106 is that it have a high breakdown voltage. That is, the voltage at which the dielectric material of the tunnel barrier layer 106 breaks down and the tunnel barrier layer 106 acts as a shorted resistance.
However, in tunnel junction devices, such as the prior memory cells 101, one problem that detrimentally effects operation of the memory cell 101 is that defects in the tunnel barrier layer 106 result in a low breakdown voltage or an electrical short. Those defects include pin holes, bubbles, surface irregularities, metal inclusions, and non-uniformity of thickness in the tunnel barrier layer 106, just to name a few.
In FIG. 2, a material for the tunnel barrier layer 106 is formed or deposited on a supporting layer 110 that can be the reference layer 106 or the data layer 102 of a magnetic field sensitive memory cell, for example. For instance, the material for the tunnel barrier layer 106 can be aluminum (Al). The material is then exposed to oxygen (O2) and is oxidized to form aluminum oxide (Al2O3). However, the prior oxidation process doesn""t uniformly oxidize all of the aluminum atoms and as a result there remains un-oxidized aluminum atoms 111 that form metal inclusion defects in the tunnel barrier layer 106. A portion of the oxygen atoms 112 remains un-reacted with the material 111 for the tunnel barrier layer 106 (i.e the aluminum); however, those un-reacted oxygen atoms 112 remain incorporated in the tunnel barrier layer 106. Similarly, some of the oxygen atoms 112 react with and oxidize a portion of a material 113 for the supporting layer 110. As a result, oxidized atoms 113 at an interface between the tunnel barrier layer 106 and the supporting layer 110 create a defect that lowers the break down voltage of the tunnel barrier layer 106.
Prior methods for depositing or forming the tunnel barrier layer 106 such as RF-sputtering, plasma oxidation, or UV-ozone oxidation ultimately leave some defects in the tunnel barrier layer 106 that result in a poor tunneling barrier with weak points therein that cause shorting or a low breakdown voltage. Moreover, the existence of those defects makes it necessary to form a thicker nitride/oxide layer for the tunnel barrier layer 106. Conversely, if the dielectric material is a really good dielectric, then the thickness of the tunnel barrier layer 106 can be reduced. A thinner tunnel barrier layer 106 also helps in improving uniformity across an entire wafer that carries multiple tunnel junction devices. A thinner tunnel barrier layer 106 also lowers absolute resistance of the tunnel junction which can be important in some applications.
However, in FIG. 2, after the prior tunnel barrier layer 106 is formed on the supporting layer 110, it is often necessary to deposit a next layer 120 in the tunnel junction stack for several reasons. The next layer 120 can be any layer necessary in the fabrication of a tunnel junction device.
First, it is desirable to control further uncontrolled oxidation of the tunnel barrier layer 106. To that end, the next layer 120 is deposited over the tunnel barrier layer 106 to cap off the tunnel barrier layer 106 thereby rendering it substantially inert to further uncontrolled oxidation.
Second, it is desirable to prevent oxidation and/or contamination of the tunnel barrier layer 106 that would result if the tunnel barrier layer 106 were exposed to atmosphere (i.e. air) by either breaking vacuum in the processing equipment or by allowing atmosphere to contaminate the ambient of the processing equipment.
Finally, it may be desirable to deposit the next layer 120 in situ as part of the fabrication process for a tunnel junction device so that the deposition of the tunnel barrier layer 106 and the next layer 120 are sequential steps in the fabrication process.
On the other hand, the aforementioned defects in the tunnel barrier layer 106 still remain after the next layer 120 is formed and those defects need to be reduced or eliminated in order to produce a tunnel barrier layer with desirable characteristics such as a minimal defect density and a high breakdown voltage, for example.
Consequently, there exists a need for a method of treating an electrically non-conductive tunnel barrier layer through an overlayer in a tunnel junction device so that an oxidation or nitridation of the tunnel barrier layer is uniform and homogenous throughout the tunnel barrier layer.
The method of the present invention address the aforementioned needs for a uniform tunnel barrier layer by exposing an electrically non-conductive tunnel barrier layer of a tunnel junction device to ultra-violet light that irradiates the tunnel barrier layer through at least one overlayer that covers the tunnel barrier layer. Oxygen or another reactant, such as nitrogen, disposed in the tunnel barrier layer from a previous processing step is activated by the ultra-violet radiation and heals out defects in the tunnel barrier layer. The ultra-violet light can be from an ultra-violet light source that is incorporated into an existing piece of processing equipment. The tunnel barrier layer can be irradiated by the ultra-violet light during the formation of the overlayer or the tunnel barrier layer can be irradiated after the formation of the overlayer. Either process can occur in situ so that potential contamination and/or unwanted oxidation/nitridation of the tunnel barrier layer or other layers of thin film in a MRAM stack or other tunnel junction device are reduced or eliminated. One advantage of the method of the present invention is that it can be easily integrated with existing microelectronics processing equipment. With the overlayer, irradiation of the the tunnel barrier layer with the ultra-violet light can occur ex situ also.
In one embodiment of the present invention, heat from a heat source is applied to the tunnel barrier layer of the tunnel junction device to further increase the activation process. As a result, more defects are removed and processing time for the activation can be reduced. The heating can occur before, during, or after the irradiation with the ultra-violet light.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention.